A conventional RFID system is used for biometric access (entrance and exit) management in an access restricted area, such as a habitable room, a factory, or an event site, management of products (physical-distribution management) in a factory or a work site, and so on. In such a conventional RFID system, management is carried out by disposing an RFID reader/writer antenna at a gate of an area to be managed, and mounting or attaching an RFID tag to each target to be managed (for example, refer to patent reference 1). Furthermore, as RFID reader/writer antenna elements, in addition to a patch antenna in which a radiating conductor which is a circular patch conductor is excited with two-point feeding using two microstrip lines, as shown in FIG. 3 of patent reference 1, there have been provided the following ones: a patch antenna in which a two-point feeding method is used, but the radiating element is a rectangular patch conductor and the feed line is a coaxial line (for example, refer to patent reference 2), a patch antenna in which the radiating element is a circular patch conductor, and the feed line is a microstrip line and a single-point feeding method is used (for example, refer to patent reference 3), and a patch antenna in which a single-point feeding method is similarly used, but the radiating element is a circular patch conductor and the feed line is a coaxial line (for example, refer to patent reference 4 and patent reference 5). As shown in patent references 1 and 3 to 5, in communications between the RFID reader/writer antenna and the RFID tag, an electric wave of circular polarization is used in most cases in order to improve the probability of the communications between the RFID reader/writer antenna and the RFID tag, because the angle of the antenna element of the RFID tag is not kept constant due to variations in the position at which the RFID tag is attached to an object.
In another conventional RFID system, a history of each product in manufacturing process stages is managed on a transportation line in an FA (Factory Automation) factory or the like (for example, manufacturing process stages of assembling products which are being carried on a conveyance system, such as a conveyor belt, and are flowing through the process stages one by one are managed), that is, the tag information in the RFID tag attached to each product moving is updated or additional information is written into the RFID tag, or the tag information is acquired from the RFID tag (for example, refer to patent references 1, 6 and 7).
[Patent reference 1] JP, 2006-113869,A (paragraphs 0017 to 0026 and FIGS. 1 to 4)
[Patent reference 2] JP, 2006-279202,A (paragraphs 0010 to 0012, and FIGS. 1 and 2)
[Patent reference 3] JP, 2006-180043,A (paragraph 0024 and FIG. 4)
[Patent reference 4] JP, 2001-237638,A (paragraphs 0038 and FIGS. 4(a) and 4(b))
[Patent reference 5] JP, 2001-52123,A (paragraphs 0011 and 0012, and FIGS. 1 and 2)
[Patent reference 6] JP, 2006-155511,A (paragraph 0071 and FIG. 2)
[Patent reference 7] JP, 2006-172101, A (paragraphs 0016 to 0020 and FIGS. 1 to 5)
In recent years, there has been a growing tendency to require product traceability (traceability) using an RFID system. In order to implement this product traceability, there is a necessity to certainly perform management of a history of each product in manufacturing process stages in each of all the manufacturing process lines or a certain number of manufacturing process lines close to the total number. Therefore, an RFID reader/writer antenna is disposed even in a process stage having a short processing line interval, though degradation of the capability of the RFID reader/writer antenna has to be avoided as much as possible and its size has to be reduced (the size of an RFID reader/writer antenna refers to the area of the electric wave radiation surface thereof unless otherwise specified) so that RFID reader/writer antennas are arranged densely and there is provided space for arrangement of processing machines and workers who perform machining. However, information reading (management) systems as shown in patent references 1 and 6 do not take into consideration the intervals among process stages and the type and size of RFID reader/writer antennas. Therefore, in a case in which, for example, an antenna having a loop shape as shown in patent reference 7 is adopted as an RFID reader/writer antenna element as shown in patent references 1 and 6, there arises a problem that in a manufacturing process stage having a short processing line interval, it is difficult to provide space for arrangement of processing machines and workers who perform machining so as to carry out management of the manufacturing processing.
On the other hand, an RFID reader/writer antenna element as described in patent references 1 to 5 is a patch antenna in which a radiating conductor (a circular or rectangular patch conductor) is arranged on a front surface of a dielectric substrate, and a ground conductor is arranged on a rear face of the dielectric substrate. In a case in which the dielectric substrate is formed to have a high dielectric constant in order to make the size of this RFID reader/writer antenna small, the size of the resonator of the patch antenna can be reduced. That is, since the area of the radiating conductor can be reduced, the size of the RFID reader/writer antenna can be reduced. In addition, the use of a coaxial feeding method of electrically connecting the outer conductor of a coaxial line to the ground conductor of a patch antenna, and electrically connecting a central conductor to a radiating conductor can achieve further downsizing of the size as compared with the use of a microstrip feeding method. There are also many patch antennas using, as their dielectric substrates, an air layer without using any dielectric material, each of the patch antennas serving as an RFID reader/writer antenna element (patch antennas equivalent to that in which a dielectric layer having a dielectric constant of 1 is sandwiched between a radiating conductor and a ground conductor).
Because a patch antenna is a resonance-type element antenna, increase in the dielectric constant of the dielectric substrate of the patch antenna simply results in increase in the Q value as a resonator and hence decrease in the radiation efficiency (the ratio of the radiation power from the antenna to the electric power supplied to the antenna). Therefore, what is necessary in order to construct a small-size good-radiation-efficiency patch antenna is just to thicken its dielectric substrate.
However, an increase in the thickness of the dielectric substrate causes an increase in the amount of variation in the thickness resulting from thermal expansion of the dielectric substrate due to variations in environmental temperature, and a stress exerted on a connecting portion at which the radiating conductor is connected to the central conductor of the coaxial feeding portion also increases. A problem is therefore that reduction of the reliability occurs in the connecting portion, for example, a crack occurs in the connecting portion when the connecting portion is soldered, and hence there is a possibility that the performance of the RFID reader/writer antenna degrades or the RFID reader/writer antenna goes out of order.
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a new RFID reader/writer antenna that includes, as an antenna element thereof, a patch antenna, that adopts, as a substrate thereof, a high dielectric constant material, that suppresses degradation of its reliability resulting from an increase in the thickness of the dielectric substrate, that implements a downsizing thereof to enable dense arrangement of plural sets thereof even in a process stage having a narrow processing line interval, and that improves a possibility of suppressing reduction in space for arrangement of processing machines using sets thereof and workers who perform machining using sets thereof.